This invention pertains to a method for injecting well-defined volumes of fluid from one channel into another at their junction in microscale devices to control cross contamination of the channels of microfluidic devices. Fluid control is accomplished generally by providing increased resistance to electric-field and pressure-driven flow in the form of a region of reduced effective cross-sectional area within the microchannels. The invention further relates to microscale devices employing these methods.
Microchannel devices are finding increasing application for separation, identification, and synthesis of a wide range of chemical and biological materials. These devices, whose channel dimensions typically range from a few microns to about one millimeter, permit miniaturization and integration of chemical and biological processes in a manner analogous to that already achieved in microelectronics. Applications for these microchannel devices include DNA sequencing, immunochromatography, analysis and identification of explosives, chemical and biological warfare agents, and synthesis of chemicals and drugs.
Microfluidic devices typically consist of two or more grooves, or microchannels, and chambers etched or molded in a substrate that can be silicon, plastic, quartz, glass, or plastic. The size, shape and complexity of these microchannels, their interconnections, and interactions influence the limits functionality and capabilities of a microsystem. In turn, the size, shape and complexity of microchannels and structures that can be used in microfluidic systems depend on the materials used and the fabrication processes available for those materials. Typical system fabrication includes making trenches in a conducting material (silicon) or in a non-conducting substrate (e.g., glass or plastic) and converting them to channels by bonding a cover plate to the substrate. The typical overall channel sizes range from about 5–100 μm wide and 5–100 μm deep.
Despite the substantial promise of these microscale systems, there have been significant drawbacks experienced in their application typically involving reduction in resolution over comparable benchscale methods. One problem that has been recognized involves sample dispersion associated with variation in fluid speed associated with fluid moving along curves or turns in the microchannel flow system. This dispersive effect arises because the fluid moving along the outer radius of a turn must travel further than that moving along the inner radius causing an otherwise flat interface or species band to be skewed. This effect is particularly pronounced in the presence of an electric field gradient, such as would be encountered during electroosmotic flow, which is greater along the shorter inner radius resulting in greater fluid speed along the shorter inner radius path.
As summarized in U.S. patent application Ser. No. 09/299,269 filed Apr. 26, 1999, entitled “Method and Apparatus for Reducing Sample Dispersion in Turns and Junctions of Microchannel Systems” and assigned to the same assignee, different approaches have been used to minimize the dispersive effect induced by the presence of curves and turns. Nordman (U.S. Pat. No. 5,833,826) utilizes focusing electrodes to obtain a more uniform electric field and hence, a more uniform flow field. However, this solution introduces increased complexity in fabrication and control of the plurality of electrodes and associated circuitry required for systems having a multitude of turns. Kopf-Still (U.S. Pat. No. 5,842,787) seeks to reduce dispersion in turns by means of channel geometries having small aspect ratios, wherein the channel depths are much greater than their widths. The smaller channel width helps reduce the difference in transit time along the inner and outer walls of a turn, thereby reducing dispersion. Dispersion can also be reduced by fabricating turns having a depth along the inner radius that is greater than that along the outer radius. This approach to reducing turn-induced dispersion would substantially increase costs since most conventional lithographic processes are designed to produce channels having a uniform cross-section.
While these approaches provide methods for reducing dispersion in a fluid sample as the fluid sample flows around curves or turns in the microchannel, they fail to address an even more fundamental problem associated with fluid flow in microchannel systems, uncontrolled fluid flow (leakage) during the operation of injecting a sample of fluid across a junction.
Numerous methods can be implemented for the transport of fluid and species (charged or uncharged) in microfluidic channels. These include: electroosmosis, electrophoresis, pressure-driven convection, diffusion, or any combination thereof. When these methods are used (alone or in combination) to inject a sample of fluid across a microfluid junction (for example, two channels intersecting in a cross), uncontrolled fluid flow resulting in significant leakage of excess injected fluid can occur. This leakage impedes the capability to inject the controlled volume of fluid (or mixture of fluids) from one stream into another stream such as would be required for accurate analysis or controlled reactions.
FIGS. 1a–1c show an example of this leakage using a typical injection device: a cross 100 with the fluid transported by electroosmosis. A dye has been added to the fluid in order to follow the path of fluid flow more easily. The fluid from which a sample is to be extracted flows in horizontal microchannel 110 under the influence of a potential gradient. When a sample is to be taken from the fluid stream, microchannel 110 is left electrically floating and a potential gradient is applied to vertical microchannel 120 for a brief period of time, in order to inject a small sample of the fluid into microchannel 120. As indicated by the pattern of trailing dye (FIGS. 1b and 1c) fluid continues to flow (leak) into microchannel 120 after the potential gradient has ceased to be applied. Electroosmotic-driven fluid flow is a ‘potential flow’ which means that fluid flow follows the paths traced by the streamlines of the electric field. Leakage occurs in this injection scheme because fluid streamlines, which correspond to electric field lines in electroosmotic-driven flow, enter the electrically floating channel. This phenomenon is graphically illustrated in FIG. 2 which shows the electric field lines at the intersection between channel 110 having an electric field contained therein and one that is floating 120. It can be seen that the electric field lines intrude a significant distance into the floating channel. This intrusion of electric field lines into the electrically floating channel not only explains the “leakage” shown in FIGS. 1b and 1c but also explains why the sample fluid is observed to enter microchannel 120 prior to application of a potential gradient to that microchannel (FIG. 1a).
The problem of leakage in injection devices has been recognized and means for mitigating this problem have been proposed. Ramsey in U.S. Pat. No. 5,858,195 and Published PCT Application No. WO96/04547 and Parce in U.S. Pat. No. 5,885,470 employed a scheme called “controlled electrokinetic material transport”, to control cross-channel leakage in microchannel systems and particularly in arrangements of integrated microchannels. In this scheme separate electric potentials are applied across the various microchannels. However, these methods require careful control of multiple electrical power sources as well as a priori knowledge of the conductive properties of all fluids in all channels to determine the required voltages. Furthermore the method is susceptible to disruption due to variations in fluid compositions, hydrostatic pressure-driven interferences, and diffusion effects, all of which may degrade the quality or purity of the injected sample.